Organic Chemistry/Introduction to reactions/Alkyne hydrogenation

Alkynes are hydrogenated in generally the same way as alkenes. However, standard catalysts like Pd/C will not allow hydrogenation to stop before the alkane stage. A "poisoned catalyst" will permit the reduction of a triple bond to a double bond by syn addition, but no further. A common poisoned catalyst is Lindlar's catalyst: Pd/CaCO3 treated with quinoline and lead acetate.

Anti addition of hydrogen atoms can be achieved with a dissolving metal reduction.

Dissolving Metal Reduction

The dissolving metal reaction takes place in a solution of sodium or lithium metal dissolved in liquid ammonia. An alkali metal is dissolved in liquid ammonia and forms a solution containing solvated electrons.

Once an alkyne is added to the solution, the electrons add to the antibonding π molecular orbital and produce a radical-anion intermediate. This intermediate has one unpaired electron and has a negative charge.

The radical anion then reacts with a proton source (generally ethanol or tertiary-butyl alcohol which is added to the ammonia) to yield a vinyl radical. The radical then accepts another electron and forms a carbanion.

The alkene is formed when the vinyl carbanion is protonated. Protonation normally occurs from the side of the double bond that is least sterically hindered, so a trans-alkene is produced.